In comparison with a tower extractor, a mixer-settler extractor is disadvantageous in that it is generally more expensive to install and the system has a greater quantity of hold-up. In carrying out an extraction operation with a smaller number of stages, however, the installation cost becomes lower, on the contrary, and the mixer-settler extractor provides quite excellent advantages for a system in which the extraction speed is low, one in which the difference in viscosities is great between oil and water and one contaminated with impurities. In a method of extracting a solute from an aqueous solution using an organic extractant consisting of a tertiary amine and an organic diluent, the viscosity of the extractant is high and, in addition the speed of the solute transfer is low, so that a tower extractor not provided with a mobile part, as represented by a perforated plate tower and a packed tower, is not generally employed. In other words, the actual height per theoretical stage becomes extremely high in these types of extractors and hence, an extractor having an extremely large tower height becomes necessary. Moreover, the operating range is narrow in order to prevent flooding and the column diameter of the tower becomes uneconomically large.
In carrying out the extraction efficiently using an organic extractant system containing a tertiary amine, therefore, it is more advantageous to use an extractor which is designed to increase the dispersion coalescence frequency of droplets, such as a mixer-settler extractor.
The problem encountered in operating a mixer-settler extractor in this system is liquid separation in the settler. If the stirring power is increased so as to improve the extraction efficiency in the mixer, the solution becomes an emulsion or reaches a state analogous to an emulsion, so that an extended period of time is necessary for the liquid separation in the settler. This results in the necessity of using an extremely large settler, with consequent increase in the quantity of hold-up and increase in the installation cost and in the quantity of initial liquid charge.
To improve the liquid separation, various methods have been employed. For example, the extraction temperature is raised or the stirring condition in the mixer is optimized to prevent the occurrence of undesirable fine droplets and to steepen the distribution of droplets diameters. Alternatively, a recycle pump is disposed between the mixer and the settler of each stage and recycles the solvent phase or the water phase so as to keep the phase ratio (of the solvent phase to the water phase) inside the mixer at an appropriate level. A method of elevating the separation speed of two liquids by adding an additive such as an alcohol, an alkali metal salt or an alkaline earth metal salt (see the Japanese Patent Publication No. 36209/1974) and an electrical method which is generally employed in a demelter of an oil refinery have also been applied with certain successes. However, the effects brought forth by these methods are not entirely satisfactory in the system to which the prevent invention is directed.